The present invention relates to a lightweight aggregate which is inexpensive and excellent in both resistance to freezing and thawing and resistance to water absorption. The present invention also pertains to a process for preparation of such lightweight aggregate.
Conventional lightweight aggregates generally involve a high water absorption, and the flash water absorption percentage thereof is known to be about 30% of the 24-hour water absorption percentage. For this reason, when such conventional aggregate in an air-dry state is used, the slump of concrete is degraded during mixing or conveying of the concrete.
To overcome such disadvantage, it is conventional practice to subject a lightweight aggregate to a prewetting treatment, i.e., a pretreatment in which the lightweight aggregate is watered so as to absorb water appropriately, before it is mixed into concrete for an actual use, thereby preventing any reduction in consistency.
However, the prewetting processing operation disadvantageously complicates both mixing of concrete and moisture control, and it is not a sufficiently reliable treatment in view of the concrete conveying system by pump and the prevention against frost damage.
To cope with the above-described problems, various types of processing method have already been proposed for the purpose of lowering the water absorption of lightweight aggregates (i.e., improving the resistance to water absorption thereof). For examples, the following methods have heretofore been known: (A) a method wherein a lightweight aggregate is rolled over a slope, and while doing so, it is subjected to curing application with a viscous material or a covering material; (B) a method wherein the surface of a lightweight aggregate is subjected to a covering treatment using a liquid solution type petrolic resin; and (C) a method wherein the surface of a lightweight aggregate is subjected to a covering treatment using straignt asphalt.
The above-described conventional methods suffer, however, from the following problems. Namely, with a preventive technique to water absorption such as the method (A), it is impossible to provide a lightweight aggregate having high resistance to water absorption that can be used in the concrete conveying system by pump wherein concrete which has not yet solidified is placed. In other words, there has been provided no lightweight aggregate which satisfies the following conditions, i.e., target values which are to be reached:
(a) To ensure a water absorption percentage of 4% or less under a pressure of 40 kg/cm.sup.2.
(b) To possess a high resistance to freezing and thawing, i.e., DF value 80 or more at 300 cycles.
When a petrolic resin material is employed as a coating material as in the case of the prior art method (B), the following disadvantages may be experienced.
First, resin materials involve the problem that, as hardening of a resin material progresses, shrinkage occurs, and this leads to generation of pinholes. It is necessary in order to prevent the generation of pinholes to laminate a multiplicity of resin layers, which means that this prior art method is impractical from the costwise point of view. It has been found that a thermal hardening resin particularly shrinks through hardening reaction and cooling taking place subsequently, resulting in a considerable internal stress.
The internal stress in the resin leads to not only lowering in the strength of the system but also generation of microcracks and pinholes.
Secondly, as will also be clear from the porous structure of lightweight aggregates, it is necessary, in order to satisfactorily impregnate a coating material into micron order pores (capillary tubes), to employ a coating material having a relatively low coefficient of viscosity and conduct operations such as pressing impregnation with a relatively large pressure difference, which fact involves increased costs, disadvantageously.
Thirdly, there is the problem of interfacial peeling caused due to the difference in terms of thermal expansion coefficient between a lightweight aggregate and a coating material. In regard to the water absorption performance under pressure in the concrete conveying system by pump, the existence of any space at the interface between the lightweight aggregate and the coating material is particularly fatal.
Fourthly, a resinous coating material takes a certain period of time to terminate hardening reaction, and this involves the problem that the tack of the coating material bonded to the surface of the aggregate remains undesirably. In consequence, there is a risk of particles of the aggregate bonding to each other in the shape of a millet and rice cake. For this reason, it is extremely difficult, with the existing technology, to effect coating of a lightweight aggregate for each individual particle thereof.
The upper limit cost of coating which is allowed for a preventive technique to water absorption is generally needed to be 50% or less of the total cost of the lightweight aggregate employed from the economical point of view. If, for example, a multiplicity of coating layers of a petrolic resin are provided on the surface of a lightweight aggregate in a plurality of steps, the cost of the lightweight aggregate is raised by 200 to 300% because of the cost of the resin and the process costs.
In addition, general resinous coating materials involve excessively high costs as materials which are used for prevention of water absorption of aggregates, i.e., even relatively inexpensive ones are 200 to 300 yens per killogram.
On the other hand, it may also be possible to employ various kinds of polymer emulsion as coating materials. However, polymer emulsions suffer from the following disadvantages:
(a) The dynamic strength of polymer emulsions is lower than those of general thermal plastic resins and thermal hardening resins.
(b) It is difficult to form a relatively thick covering layer.
In addition, since water (generally about 50%) contained in a polymer emulsion is stored for a long time in the aggregate by the covering layer formed on the surface of the aggregate, the water may have a bad influence on a freezing and thawing test.
A preventive technique to water absorption using straight asphalt such as the method (C) also has the problem that it is not possible to attain prevention of water absorption which is satisfactorily effective under a pressure of 40 kg/cm.sup.2 in the concrete conveying system by pump.